There has been used an ultrasonic sensing system for informing a vehicle driver of the approach of an obstacle such that when a vehicle driven by a driver is approaching an obstacle in his/her surroundings while the vehicle is traveling, an ultrasonic sensor attached to a bumper or the like of the vehicle senses the approach of the obstacle, and then issues a warning alarm such as a buzzer sound to the driver. As a retainer for attaching such ultrasonic sensor that constitutes this system to a vehicular bumper or the like, there is known, for example, a retainer disclosed in Japanese un-examined patent application publication No. 2016-013812 (hereinafter patent document 1). The retainer includes a retainer main body through which a sensor main body of the ultrasonic sensor is inserted, and attaching portions for attaching the retainer to the bumper. Each of the attaching portions is connected to the retainer main body via a connection portion and a base portion, having, as a whole, a substantially rectangular shape in a plan view.
Also, there is disclosed in Japanese patent publication No. 4056568 (hereinafter patent document 2), another retainer (or retaining part) which is tube-shaped in order for an ultrasonic sensor to be inserted thereinto. This retainer has an end face as a front part thereof such that the end face and a bumper are fixed to each other by adhesion or the like.